Vintner's Collective on Main Street in Napa, Calif., sustained major damage to its facade, but little inside damage according to the owner after a strong earthquake hit the San Francisco Bay Area centered near American Canyon, on Sunday, August 24, 2014.

Spectators survey the damage to a building at the corner of Brown Street and Second Street in Napa, California, after an earthquake measuring 6.0 on the Richter scale struck in the early morning of August 24, 2014.

Photo: Alvin Jornada, Special To The Chronicle

Spectators survey the damage to a building at the corner of Brown...

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The Goodman Library building shows damage from an earthquake in Napa, Calif., as a strong earthquake hit the San Francisco Bay Area centered near American Canyon522298, on Sunday, August 24, 2014.

The facade of a building at Second and Brown Streets in Napa, Calif., shows severe damage after a strong earthquake hit the San Francisco Bay Area centered near American Canyon, on Sunday, August 24, 2014.

Bricks are in the street after a building was damaged during an earthquake in Napa, Calif., Sunday, Aug. 24, 2014. A large earthquake rolled through California's northern Bay Area early Sunday, damaging some buildings, knocking out power to thousands and sending residents running out of their homes in the darkness. (AP Photo/Ellen Knickmeyer)

The destructive earthquake - the strongest in the Bay Area since the 1989 Loma Prieta quake - served as a stark reminder about long-running concerns regarding aged state infrastructures, particularly vulnerable older unreinforced masonry buildings that continue to pose a threat to California residents.

"It most definitely is a wake up call ... to realize what the danger of this is, and what the risks are," said Emir Macari a member of the state Seismic Safety Commission and a professor of seismic safety at Cal State University Sacramento.

Macari estimated that nearly a quarter of a century after the Loma Prieta quake, there still are "tens of thousands" of unreinforced masonry buildings in California.

Most of the regulation of such structures is left up to local communities, but the Napa quake is a reminder that enforcement issues vary widely and remain a "gray area," he said.

In California, there's long been concerns that statewide regulation and mandates "would be overwhelmingly expensive, and something insurance companies wouldn't cover," he said. There are fears that building owners would have to vacate ... it would be a huge economic loss for the state."

A major problem is that some of the unreinforced masonry structures are quite technical to reinforce, he said, and require costly retrofitting such as internal bracing.

In Napa itself, city officials admitted that not all the city's property owners had complied with a 2005 city ordinance calling for seismic upgrades to unreinforced brick buildings, despite a 2009 deadline that's long since passed.

A "small handful" of buildings never met the new earthquake codes, Rick Tooker, Napa's community development director, said at a news conference. He added that the city was "actively pursuing" the property owners who didn't make the upgrades.

At least three of the city's unreinforced buildings were badly damaged by the quake, Napa City Manager Mike Parness said at a noon news conference.

"Of those that had major damage, the three buildings on Brown Street," numbers 816-820, all "had not been brought up to standards," Parness said. He estimated 15 or 16 buildings had been red-tagged.

But the work that has been done since the Loma Prieta earthquake has made a real difference to the city, he said.

"I think it's paid off," Parness said.

State and local officials already are working together to get a complete assessment and picture of building safety in the quake's wake, Macari said.

Statewide, such inventories of potentially dangerous unreinforced buildings "need to continue ... and the (Napa) assessment is going to give us a lot of information," he said.

The Napa quake reprises concerns about unreinforced masonry, even though such construction has been banned in California since 1930.

Those laws were significantly strengthened in 1986, when California passed a law requiring hundreds of local governments to inventory such buildings, establish a risk reduction program and report to the state.

In the Loma Prieta quake, a 6.9 magnitude event, 374 of the 2,400 unreinforced masonry buildings in the region were damaged so severely they had to be vacated, according to the Federal Emergency Management Administration.

Local officials already are looking toward the next big quake to hit the Bay Area, something experts say is not a question of if, but of when.

In San Francisco, Mayor Ed Lee said in a statement that the city has made major efforts to invest in and upgrade its emergency services system.

"It is critical that our water supply systems, our police and fire stations and our hospitals can rapidly respond in an emergency," he said. "This is a good reminder that we need to do what can now, before the next earthquake, because that will make our city's recovery all the more effective."

Disasters "can happen at any time with little or no warning," he added. "That is why it is important to take steps now so we are ready for any emergency."

Rep. Adam Schiff, D-Burbank, issued a statement calling the UC Berkeley's Seismological Lab's 10-second warning provided to Bay Area residents before the Napa quake a "successful use of the early earthquake warning system." He said it showed that "a full West Coast system would be successful in giving residents, businesses and critical infrastructure" a heads-up in the event of a large magnitude quake.

Other government officials were working to deal with the quake's aftermath. Gov. Jerry Brown quickly declared a state of emergency after the temblor damaged roads, homes and businesses, as well as water and gas lines.

The state's Office of Emergency Services "has been on full activation since early this morning," the governor said in a statement, "and is working closely with state and local emergency managers, first responders and transportation officials to respond to impacts to residents and critical infrastructure.

Mark Ghilarducci, the state's director of emergency services, said his agency still was trying to gauge the extent of the devastation.

"We're working with the local governments, getting assessments of the total amount of damage," he said in a statement, as well as responses needed for fire, rescue, and emergency medical service in Napa, Solano, Vallejo, and Sonoma counties.

The agency has been dispatching mutual aid, checking with officials in and around Napa to make sure communications have been re-established, and that "people who have been impacted by the event are taken care of."

The 6.0 quake "felt like a magnitude 7" because of the geography of the area, Ghilarducci added, warning that residents should be ready for more quakes.

"There's going to be a number of aftershocks in the area," he said, "So they need to be prepared for that ... you need to duck, cover, hold on to something heavy and hang on."